Frequency control



Jan. 11, 1938 c, w HANSEL! 2,104,801

FREQUENCY CONTROL Original Filed Oct. 4, 1933 3 Sheets-Sheet 1 SOURCE FREQ.

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ATTORNEY 'Jan. 11, 1938. c. w. HANSELL FREQUENCY CONTROL Original Filed Oct. 4, 1933 5 Sheets-Sheet 2 60 C 76!! SOURCE ig. lb

70 FREQUENCY COR/75C 77/1/6 MEANS AUDIO TRANSFORMERS D. 6. POWER SOURCE INVENTOR C.W.HANSELL K? I ATTORNEY Jan. 11, 1938. c w HANSELL 2,104,801

FREQUENCY CONTROL Original Filed Oct. 4, 1955 5 Sheet-Sheet a INVENTOR 0. W. HANSELL ATTORNEY Patented Jan. 11,1938 I 1 UNITED STATES PATENT OFFICE FREQUENCY CONTROL Clarence W. Hansell, Port Jefferson, N. Y., as- I signor to Radio Corporation of America, a co poration of Delaware Original application October 4, 1933, Serial 1! 692,092. Divided and this application September 17, 1934, Serial No. 744,273

12 (Claims. (Cl. 250-36) This case is a division of my copending applicazero beat, there is a reversal in phase oi the retion Serial No. 692,092, filed October 4, 1933. sulting beat. Figures la and 1b and Figures 2 For automatic frequency control it has beenand 3 are alternative circuits which may be used proposed heretofore to beat a controllable frein the system illustrated in Figure 1.

5 quency source against a constant frequency Turning in greater detail to Figure l, have 5 source. The resultant frequency differentiated provided a constant frequency source i2? which beat is passed through two filters in such a way may be a piezo-electric crystal controlled, and that if the beat, and consequently the controllable incidentally, temperature controlled oscillation frequency, is of correct value, no potential difgenerator so designed as to be substantially conference will arise across the filter circuits. Howstant in frequency. Since large power output is 0 ever, if the beat shifts in frequency, the potential not essential the oscillator I22 may be relatively difference across the filters is utilized to vary the small and, consequently, inexpensive. The outfrequency of the controllable oscillator so that the Dill; om the crystal controlled source 822 is fed oscillator is shifted in frequency in such a. way as through O epa a e cui s to the int produce th desired t, This ystem, howput electrodes of two vacuum tube power detectors 15 ever, suffers from the disadvantage that it is or rectifiers I30, I32. In one of the circuits 24. liable to produce the correct beat frequency note and as s a in C c iii, there is p when the controllable oscillator is either above or vlded phase Shifter 534 which y a timed below the desired frequency by the same amount, circuit, a radio goniometer or an artificial trans- Consequently, with such an arrangement, con- 5 01! e. O v o s y, t e Constant frequency fusion is likely to ensue since an operator cannot so e in y i cl de eque c multipliers o readily tell, with such an arrangement, whether that the Output q n y corresponds in or not he is operating above or below his conqu n y 0 t f qu at whichitis dth t stant frequency comparing wave. Therefore, a h generator 0 controllable frequency be further object of my present invention is to pro- Operatedvide a frequency correcting and controlling sys- Rather than use a phase shifter the two cirtem which shall have only one possible frequency cuits I24, I26 may be tuned by conde se s 3 of operation. I40 in which case the phase shift may be obtained In carrying out this angle of my present inby simply adjusting one circuit above and the vention, I have discovered that the beat between t r be ow the output equ n y s t at n rso waves of nearly the same frequency reverses in ries the current of leading phase and the other polarity or phase as the Waves are passed through es ag g Cu e t pe to the Output zero beat. This reversal of phase of the beat bevoltage of the source I22. The phase shift should tween energies of nearly like frequency as they preferably be made such that the current fed are passed through zero beat is utilized to adjust from circuits I24, I26 into "the input circuits l42,35 the frequency of a controllable oscillator so as I 44 of the detectors differ by from 30 to 90 deto bring it back in frequency to correspond to grees. I may also shift the phases by means of that of a highly constant frequency wave. reactance and resistance circuits.

In connection with the frequency control of an The detectors I30, I32, while shown to be of the oscillation generator, it is to be noted that a. certhree electrode type may be of the diode or four 40 tain amount of leeway is permissible and, in fact, or five electrode types and, in fact, may be reit is desirable that an oscillation generator be not placed by other forms of detector such as the corrected in frequency for every slight change copper oxide type. i which may occur since this would tend to subject To the detector input circuits I42, I 44, there 45 the correcting apparatus to incessant operation is also supplied from coils I46, I48 connected to 45 with its concomitant undue wear. A further feathe controllable source I35 energy from the conture of my improved frequency controlling system trollable generator. The detectors I30, I32, res des in the provision of an arrangement which therefore, produce in the low frequency transdoes not become operative to exercise its freformers I50, I52, beats between energy from the 5 qucncy control untila drift in frequency of apreconstant frequency source I22 and the source 50 determined amount has taken place. I35 to be controlled. It is to be noted that source Figure 1 is a schematic circuit diagram of 2. I35 is to be operated at substantially the same circuit for frequency control which, for its operafrequency as that of the output of the constant ticn, depends upon my novel teaching to the effrequency controlling source I22. feet that as two oscillators are passed through The beat frequency output of the two detectors I30, I32, will have a phase relation corresponding to the difference in phase between the currents in the two radio frequency circuits which couple source I22 with the inputs to the two detectors I30, I32. If, for example, condensers I38, I40 are set to give a phase difference of 60 degrees between inputs from source I22 to the two detectors, then, in the outputs of the two detectors will appear beat frequency energy with a frequency equal to the difference between the frequencies of sources I22 and I35 and having a phase difference of 60 degrees.

The detectors, while shown of the bias type, are preferably in practice made to have grid leak resistors and condensers so as to eliminate the necessity of the grid biasingsource. The outputs of the detectors, namely, the energies appearing in the secondaries of transformers I50, I52, are applied to the input or grid filament circuits of two vacuum tubes I54, I56 having a common output circuit including the electromagnetically operated relay system H6, H8.

When the generator I35 of controllable frequency is operating at exactly the output frequency of source I22, the beats appearing in transformers I50, I52 will be of zero frequency and double the frequency of either of the sources I22, I35. The double frequency, however, will be shunted around the transformers I50, I52 by the action of lay-passing condensers I56, I60 and the switch member II8 will not be moved in either direction. However, with a shift in frequency of I35 away from the output frequency of controlling source I22 the direct plate currents of the two vacuum tubes I56, I54 will be unequal due to the phase relations existing between their grid voltages and thus any inequality in the plate currents is utilized to operate the relay system H6, H8 which in turn may operate an alarm, or operate a reversible motor for automatically correcting the frequency of the controllable source. The principle upon which the system of Figure 1 operates may briefly be summarized by stating that as the generator I35 passes through zero beat, the phase relationship of the resultant beat frequency energies appearing in transformers I52, I50, reverse or shift from one polarity to another. This phenomenon, namely, reversal in phase of the beat as the frequencies combined pass through zero beat, is utilized to cause operation of the system to cause a return to zero beat condition and hence substantially identical frequency of operation of the controllable source with the controlling constant frequency source.

To understand the operation of the device shown in the figure, it should again be noted that the polarity of the beat frequency outputs from the two detectors is reversed as the transmitter frequency I35 is varied through zero beat with respect to the output of the crystal unit. Consequently, if the transmitter frequency is above the crystal frequency, one of the vacuum tubes, say I56, will have its grid swung positive in advance of tube I54 by an amount of time corresponding to the beat frequency and the phase relation between the two grid voltages. If the transmitter frequency is below the crystal frequency by a like amount, the polarities are reversed and, consequently, the grid of I54 is the first to swing positive instead of the grid of vacuum tube I56.

Under these conditions, it is only necessary to provide some method for making the direct plate current of one vacuum tube greater than that of the other in accordance with the phase relations between their grid voltages. One method of doing this involves the use of a condenser and resistance such as are shown at I62. Assume that the time constant of condenser and resistance I62 is such that beginning at, say cycles, the condenser begins to cause an appreciable smoothing out of the voltage variations across the resistance. Then the vacuum tube whose grid first swings positive will have grid rectification which will charge up the condenser and cause a negative potential to exist on the tube at the time when it swings most positive. Assume, for example, that the phase relations between the two tubes is 60 degrees. Then if the excitation to I56 is leading in phase, I56 will have its grid potential positive once each cycle and I54 will follow at 60 degrees or one-sixth of a cycle later. It will then be five-sixths of a cycle before I56 again swings positive. Consequently, the time for the condenser to lose its charge after I56 swings positive, is only one-fifth as great as the time allowed for the charge to leak off during the time which elapses between I54 and I56 swinging positive. Consequently, tube I56 with its leading phase, will have an effective bias potential less negative than I54 and its plate current will consequently be greater than I54. If the polarities are reversed, I54 will be closely followed by I56 and there will be a relatively long time interval between positive peaks on I56 and I54 and, consequently, I54 will have a lower average bias potential and its plate current will predominate.

Thus it may be seen that the time discharge rate of condenser and resistance I62 in combina tion with the phase relations in the excitation between the two vacuum tubes, will give a differential plate current variation as the transmitter frequency is varied through zero beat with respect'to the output of the crystal oscillator. If the beat frequency increases, the variations in potential across circuit I62 will tend to be smoothed out and reduced to smaller and smaller values. This would result in the differential variation decreasing at relatively high audio frequencies and would limit the maximum operating band in which the tube currents could be used to indicate in which direction the transmitter had varied. The frequency band may be increased by using a second resistance condenser circuit I66, connected in the plate circuit of the tubes. This circuit may be adjusted to have a different time constant than that of circuit I62. In this case it may be assumed that the condenser of circuit I66 begins to cause a lag in the potential across the resistance at a point where the audio frequency is so .high that circuit I62 is beginning to lose its effect. In other words, the operating ranges of circuits I62 and I66 are made to overlap in such a way as to obtain almost twice as large an operating band as could be obtained with one circuit.

To still further widen the operating band, additional circuits of still different time constants such as I68, I10, may be serially connected in the common grid lead I12 and also in the plate circuit, in series with I66, if desired. In addition to the time constant circuits I62, I66, etc., the relay coil II6 may have considerable inductance and operate as an audio frequency auto transformer or, if expedient, two windings of a transformer may be connected in series with the two sides of the relay in such a way that their magnetic fields are adding in a common core.

This transformer action may be utilized to extend the differential action of the two tubes still further. For example, assume that the excitation to tube I56 leads I54 by 60 degrees. Then when tube I56 has its current increasing it will make the plateof I54 more positive, but this will not cause a current to flow in I54 because at this time its grid potential is still negative. However, at the time when the current in I56 is decreasing this will make the plate potential of tube I54 lower at a time when its grid is positive and the plate current will, consequently, be less in I 54 than it-otherwise would have been. Thus, the action of the time constant circuits and the transformer action of the relay all tend to cause the tube with the leading phase to have a higher plate current than the other tube. If the transmitter frequency moves through zero beat in such a way as to make the excitation to I54 lead that of I56, then I54 will have a predominating plate current and the relay will be reversed.

In order that the operator may have an indicator to show which way the frequency is varyingbefore the relay has operated, a double range direct current voltmeter I14 may be connected across the relay coil. The direction of reading in this meter will then show whether the transmitter frequency is above or below the correct value. If preferred, two separate direct current meters I76, I18 may be used in series with the plate circuits of the two vacuum tubes, and further a bias source may be connected in series with lead I'I2 to make the tubes initially cut off. Deflection of either meter I16, I78 will indicate leading phase applied to its corresponding vacuum tube.

The motion of the relay II8 under the coni'rol of tubes I54 and I56 will serve to close one contact or another, to operate an alarm system or to run a small motor in one direction or another to correct the transmitter frequency.

To summarize the operation of the frequency controlling arrangement illustrated in Figure 1,

it is to be remembered that source I35 is to be controlled or operated within quite narrow lim its, at the same frequency as the frequency of oscillation of source I22. When both sources, namely, I22 and I35, are of the same frequency, there will be zero output from the detectors I30, I32 as a result of which there is either no current or uniform current flow through the tubes I56, I54 and the tongue I I8 is not moved to either direction. Should generator I35 drift in frequency above that of source I22 the relative phases of the resultant beats from the two detectors will differ in one direction and should the generator I35 fall below in frequency with respect to the frequency of source I22, the phase of the resultant beats will differ in the opposite direction. That is, the polarity of the beat frequency output from the two detectors I30, I32 is reversed as the source I35 passes through zero beat with respect to the output of the crystal unit I22. Now, assuming that the beat in the secondary of transformer I52 leads in phase the beat in the secondary of transformer I56, tube I56 will be caused to draw more current first and incidentally impressing anegative voltage upon the condensers in the common grid circuit for both tubes I56,.I54. A time later, when tube I54 is operated there will have been impressed from the previous operation of tube I56 a negative potential on the grid of tube I54 as a result of which with a continuation of the condition just assumed, the mean current flow through I56, phase, tends to rob the other tube of current to make tube I56 will be greater than that through tube I54. In other words, that tube of the two tubes I54 which has the beat applied in leading causing the tube experiencing the leading phase excitation to draw a greater average current than the other tube. Asshown in Figure 1, the greater current is caused to operate a relay tongue in one direction or another, in turn causing a reversible motor to turn in one way or another and preferably in such a way as to cause the generator of controllable frequency to be adjusted so as to return to a frequency corresponding exactly with the frequency of the constant frequency source I22 and so that there is substantialy zero beat between the controllable source I35 and the controlling source I22 again.

It will, of course, be possible tomake many modifications in the detail arrangements of Figure 1 for carrying out this important aspect of my present invention. For example, the detector tubes may be of the grid leak type instead of the power type as shown. The output transformer for the detectors may be made of relatively low exciting reactance, if desired, so that the audio frequency outputs to the first detector tubes are kept small until the transmitter frequency has varied more than a predetermined amount. .For example, it may be desirable not any correction of the transmitter frequency unless it varies more than 250 cycles from the correct value because this degree of control may be suflicient for communications purpose and to attempt to hold the transmitter any closer would result in unnecessary wear on the frequency correcting mechanism.

Also, in the diagram I have shown batteries for supplying anode and grid potentials whereas in actual practice, rectifiers or motor generators would commonly be used. It will also be apparent that for the sake of simplicity, I have omitted the circuits for supplying filament heating energy. In practice, the filament heating might be from either a direct current or alternating current source and probably an alternating current source would be used.

In practice. I may combine the functions of tubes I54, I53 with those of I36, I32 by applying the condenser-resistance circuits I10, I68, I62, I66 and the relay coil II6 to the grid bias and anode supply circuits of the detector tubes I36, I32. I have shown two separate pairs of tubes in Figure 1, for carrying out the functions of radio frequency detection and audio phase detection, in order to make the scheme easier to comprehend. Obviously, combining both functions in one pair of tubes will result in economy in equipment.

In place of two ordinary vacuum tubes such as I54 and I 56, we might use vapor electric devices sold under the name of 'I'hyratrons" or Grid glow tubes. These devices, which are struck into action by grid excitation, but are extinguished only by reduction in plate voltage, are supplied with anode energy from a 60 cycle source with circuits such that the first tube to have its grid swung positive would become a short circuit for a half cycle of the 60 cycle energy and would short circuit the anode supply of the lagging tube to prevent it breaking down at all. In this case, the leading tube would carry rectified current and the lagging tube would remain an open circuit. Such a circuit has been illustrated in Figure 1a which shows Thyratron or grid glow tube circuits suitable for carrying out the same functions as tubes I54, I56 and their associated circuits in Figure 1. It should be noted that the circuits of Figure 1a are not limited in application to radio equipment, but may also be used in audio or power equipment to indicate phase relations. For example, in power generating stations the tube circuits may be utilized in performing most of the functions ordinarily performed by synchroscopes, even including the closing of circuit breakers to connect generators to the power system at the instant the generators have the same phase as the power system. A disadvantage, though not a very important one, of using vapor devices is that it would not be convenient to provide a voltmeter or other instruments to indicate to an operator the drift in frequency before the relay operated so that he might quickly make manual adjustments to prevent operation of the alarm or automatic correcting device.

In greater detail concerning Figure 1a, the beats to be compared are fed as before to the grids of vapor electric devices T1, T2. The tubes T1, T2 are biased by the C battery arrangement shown which may be replaced by a source of rectified alternating current, such as a potentiometer resistance across a copper oxide rectifier. Whichever tube is struck by leading phase grid potentials will cause an are or glow discharge through either tube T1 or T2 and the voltage drop produced in the common plate resistance R will prevent current flow through the other tube as its grid swings positive a short time later. It is to be noted that in the system of Figure 1a., alternating current potentials are used for energizing the plate electrodes. This is done in order to periodically break the plate current through the tubes, otherwise, with direct current on the plates, and with one tube drawing current, that tube would no longer allow of desired operation as it would continue to keep the other tube extinguished even though, later on, leading voltages were applied to the other tube.

Still another very useful and practical circuit to detect phase changes is illustrated in Figure 1b. In Figure 1b, I have shown a tripping circuit, described by James L. Finch in his U. S. Patent 1,844,950 which consists of a pair of multielectrode vacuum tubes TTi, 'ITz, inter-linked by means of resistance coupling circuits RI, R5, R6, R1, in such a way that if either tube predominates in carrying plate current it will cause the other tube to cut off and its own current to be a maximum. If this condition of current through a single tube is disturbed by momentarily forcing' current to flow in the tube which had cut off and at the same time decreasing the flow of current in the tube which had been carrying current the unbalance is reversed, that is, the action of the two tubes is somewhat'analogous to a snap switch the contacts of which can have only two definite positions and which is so designed that any intermediate positions are unstable and cannot be held for any appreciable time. By applying the phase displaced detector outputs to the two tubes, it is possible to cause these detector outputs to throw the unbalance of the tripping circuit back and forth. The output of one detector can be used to cause one tube to take all the current while the output of the second transformer can be used to cause the second tube to take all the plate current. The phase displaced output from the two detectors will cause first one tube and then the other to carry plate current, but the interval during which one tube carries current will be much greater than the interval available to the other, depending upon whether or not the transmitter whose frequency is bein monitored is above or below the frequency of the standard monitoring source. Due to the unbalance in time during which the tubes carry current there will be an unbalance in average direct current which can be utilized to operate the relay RY.

In practice, it is very desirable that the tripping circuit be made inoperative unless there is an output from the two detectors and unless the frequency of this output is above a predetermined value. I obtain such an action by normally biasing the two tripping tubes so highly that no plate current can flow in either of them and then overcome this bias or a. portion of it by means of another direct current voltage obtained by rectification of a portion of the detector outputs. Thus, I have shown oxide rectifier OR in the sketch to obtain the bucking voltage for the bias source. Since the amplitude of the bucking voltage is proportional to the amplitude of the audio output from the detectors the whole system is relatively insensitive to variations in volume of detector output. This is a desirable feature for practical applications where constant volume would be somewhat difficult to obtain. In the arrangement which I have shown it is only necessary for the volume to be sufficiently high to cause the tripping circuit to function. If either the standard frequency source or the transmitter should fail the tripping tubes are immediately rendered inoperative and no further changes are made in the adjustment of the transmitter frequency control equipment. Consequently, as soon as the failure or interruption of energy is corrected the transmitter frequency immediately comes again under the control of the correcting device and in no case has been run to the extreme range permitted by the frequency control. Also, if the device is used to monitor the output from a keyed transmitter, its operation is not effected by the keying except, of course, that the relay may tend to make contact intermittently if the keying is quite slow.

If desired, the plate currents of the vacuum tubes or vapor devices may, of course, be used to directly operate an alarm device or to control the power supply to a motor without the intermediate control relay. For example, the plate currents of the two tubes may be passed through field coils of a small motor in directions to cause opposing fields and in this case the direct resultant field magnetization will depend upon the difference between the two tube anode currents.

Further circuits for detecting the phase of the beat by .comparing two like frequencies one of which tends to drift through zero beat, are illustrated in Figures 2 and 3. The circuits of Figures 2 and 3 illustrate means for increasing the speed of detection of the phase shift.

' Turning to Figure 2 in greater detail, the input to the tubes I54, I56 corresponding to similarly labeled tubes in Figure 1, is derived from transformers I 52, I50 in turn connected to detectors not shown corresponding to the detectors I30, I32 of Figure 1, one of the inputs in either transformer-I52 or I50 is leading in phase depending upon whether the frequency to be monitored is above or below the standard frequency from the frequency controlling source. The condenser resistance circuit I80 and resistors I84, I86 and the feed back through condensers I88, I90, together with the action of transformers I92, I94 all tend to produce an unbalance in plate currents of the two vacuum tubes I, III. The current through one or the other tube predominates, of course. when the phase relations of the input from the detector output transformers I50, I82 reverses. The unequal plate currents then cause operation of the electromagnetic relay whose output leads I20 may be connected to any suitable alarm or frequency correcting device such as a frequency correcting motor.

The transformers I 82, I of Figure 2 are wound upon one core Ill and so arranged that the transformers I50, I52 of Figure 3. As before the condenser-resistance combination I is charged first by that tube to which the leading phase currents are applied. The retained charge upon the condenser-resistance combination I80 therefore tends to reduce the current flow through the tube to which the beat is relatively lagging in phase. The transformers I80, I98 are so arranged and connected that as the plate current through that tube to which the leading phase voltages are being applied decreases, the induced electromotive force caused by the decrease in current is such asv to reduce the voltage on the grid of the tube being subjected to the lagging beat. In this way the unbalance due tothe leading beat is accentuated, causing quicker and more positive operation of the alarm or frequency corresting device relay 8, H8. In the arrangement shown in Figure 3 the grid connections to the transformers I96, I98 may be reversed so that leading voltages applied to one tube in effect causes the other tube to draw current and, hence, operate the electromagnetic relay H6, H8.

In circuits similar to that shown in Figure 3, I may, of course, use transformer coils I96, I98 to feed back to the screen grid of a four electrode tube instead of to the control grids of three electrode tubes as shown. In fact, I may employ tubes with any number of electrodes in any of the circuits of my invention. I may also use tubes with any type of cathode and, in practice, heater type cathodes, which permit the use of alternating heating current withoutdisturbance in the system, are preferred.

The frequency controlling system of Figures 1, 2, and 3 may vary a condenser or an inductor or a combination of both for controlling other forms of oscillation generators such as used, for example, in heterodyne receivers or elsewhere.

Having thus described my invention, what I claim is:

l. The method of making a generator of controllable frequency substantially follow the fre: quency of a controlling source of substantially like frequency which includes combining two portions of energy from said controllable generator with two phase displaced portions from said source to produce a pair of phase displaced beats, the relative phases of which severse when the frequency of said generator exceeds said source and vice versa, and, utilizing the reversals in phase of the beats produced when said beats exoeed a predetermined substantial frequency, to alter the frequency of the generator so as to substantially correspond in frequency to the frequency of the controlling wave.

2. The method of making a generator of controllable frequency substantially follow the frequency of a controlling source ofsubstantially like frequency which includes combining energies.-

from said controllable generator and source to produce a pair of phase displaced beats, and, utillzing, only after there has been a predetermined substantial departure infrequency of said controllable generator from said controlling wave, the reversals in phase of the beats produced to alter the frequency of the generator so as to correspond in frequency to the frequency of the controlling wave.

3. In a frequency controlling system, the combination of a source of constant frequency energy, a source of variable frequency energy of a frequency substantially equal to the frequency of energy from said constant frequency source, means for producing a pair of phase displaced beat notes, the relative phases of which are characteristic of the relative frequencies of said sources, and means for utilizing reversals in phase of said beats when said sources pass through said zero beat by a material amount only to vary said 0 source of variable frequency in such a direction as to bring it closer to the frequency of said con stant frequency source.

4. A frequency controlling system comprising a source of constant frequency energy, a source of controllable frequency energy corresponding-substantially in frequency to the frequency of said constant frequency source, means for producing a pair of phase displaced beats with energies from said sources, and means for utilizing reversals in phase of said beats to correct the frequency of said controlling source so as to correspond in frequency to the frequency of said constant frequency source, said last mentioned means being operative only after a drift in frequencies between said sources of at least 100 cycles occurs and being inoperative until said predetermined drift in frequency has been reached.

5. The method of holding the frequencies of two energy sources closetogether which includes producing polyphase' beats between the two energies, the relative phase relations of which indicate which of the two energies has the higher frequency and then utilizing the relative phase relations of the polyphase beats to actuate means for bringing the frequencies of the two energy sources closer together when said sources differ in frequency by a material amount.

6. The combination with an oscillation generator including frequency controlling means, of means for actuating said controlling means to maintain the oscillations of said generator at substantially constantfrequency irrespective of any tendency of said oscillations to shift in phase or frequency, which consists of a source of oscillations of substantially fixed frequency, electron discharge devices for demodulating superimposed oscillations from both of said sources, the relative frequencies of which may shift to obtain beat notes of relative phases characteristic of the relative frequencies of said generator and source, coupled by separate circuits to said source of oscillations of substantially fixed frequency and with said oscillation generating means, an additional circuit coupled with said electron discharge I cuit coupled to said additional circuit to be energized by energy therein and coupled to said frequency controlling means, and means in said additional circuit for preventing the same from energizing said actuating circuit until said generator and source of oscillations are of materially different frequencies and produce beat notes of considerable frequency.

7. A frequency controlling system as recited in claim 4 wherein means is connected with said producing means and with said second named means to render the latter inoperative on the failure of either of said sources.

8. In a frequency controlling system, an oscillation generator including frequency controlling means, a source of constant frequency oscillations of a frequency substantially equal to the frequency of said generator, a pair of electron discharge devices each having a control grid and a cathode and an anode, circuits coupling said source of constant frequency oscillations to the control grids and cathodes of said tubes, circuits coupling said oscillation generator to said control grids and cathodes an additional pair of discharge devices each having a control grid, a cathode and an anode, transformer means coupling the anodes and cathodes of said first named pair of electron discharge devices to the control grids and cathodes of said additional pair of discharge devices, said transformer means being responsive only to oscillations above a predetermined frequency, a relay connected with the anodes of said last named pair of discharge devices, and an actuating circuit coupling said relay to the frequency determining means in said generator.

9. A system as recited in claim 8 wherein a capacity shunted by an impedance is connected between the control grids and cathodes of both" of said additional discharge devices.

10. In a monitoring system, a tunable source of wave energy the frequency of which is to be controlled, a source of constant frequency wave energy of the same order of frequency as said first named source, means connected with both of said sources for producing phase displaced beat notes the phase relations of which reverse as the frequency relation of the said sources changes, means for producing a direct current the direction of which is characteristic of the phase relation of said beat notes when the frequency of said beat notes exceed a substantial value, and means for re-tuning said controllable source in accordance with the direction of said direct current to bring said beat note frequency to substantially zero.

11. In a monitoring system, a tunable source of wave energy the frequency of which is to be controlled, a source of constant frequency wave energy of the same order of frequency as said first named source, means connected with both of said sources for producing phase displaced beat notes the phase relation of which reverses as the frequency relation of the said sources changes, means for producing currents the direction of which is characteristic of the phase relation of said beat notes, means for indicating the relative intensities of said currents to thereby indicate the relation of said phases, means for producing a resultant of said currents when the frequency of said beat notes exceeds a substantial value, the direction of which resultant is characteristic of the relative phases of said beat notes, and means operated by said resultant current for re-tuning said controllable source to bring said beat note frequency to substantially zero.

12. In a system for controlling the frequency of a source of high frequency oscillations having a tuning means, the frequency of which source may vary, a source of wave energy of substantially like frequency which is constant, a pair of electron discharge devices each having a control grid. a

cathode and an anode, means for applying wave energy from said source of oscillations which may vary, to the control grids of said tubes, means for applying wave energy from said source of wave energy of constant frequency in phase displaced relation to the control grids of said tubes, means for biasing the control grids of said tubes relative to the cathodes of said tubes to potentials such that said tubes operate as rectifiers, transformer means coupled to the anode electrodes of said tubes, said transformer means being of low exciting reactance whereby the secondary windings thereof are energized only in the presence of beat notes above a predetermined frequency, a pair of thermionic discharge devices having their input electrodes coupled to said secondary windings and their output electrodes coupled to indicating means and to relay means, and a connection between said relay means and the tuning means in said source of oscillations of controllable frequency.

CLARENCE W. HANSELL. 

